Field of the Invention: The present invention generally relates to abrasive compacts comprising a polycrystalline diamond layer and a cemented carbide support. More particularly, the present invention relates to a method for making such compacts which substantially eliminates cobalt depletion from the carbide support during high pressure/high temperature processing, and the products made thereby.
Prior Art: Polycrystalline diamond tools suitable for use in applications such as rock drilling and machining are well known in the art. U.S. Pat. No. Re.32,380 describes composite compacts comprising a polycrystalline diamond layer in which the diamond concentration is in excess of 70 volume percent and wherein substantially all of the diamond crystals are directly bonded to adjacent diamond crystals, and a cemented carbide support material which is considerably larger in volume that the volume of the polycrystalline diamond layer. Typically the carbide support is tungsten carbide containing cobalt metal as the cementing constituent.
The '380 patent teaches that the cobalt contained in the carbide support or carbide molding powder makes itself available to function both as the metal bond for sintering the carbide and as a diamond-making catalyst required for conversion of graphite to diamond. Although compacts made according to the process of the '380 patent are suitable for most purposes, the unregulated infiltration of cobalt from the carbide support into the diamond layer leaves an excessive amount of cobalt among the diamond particles, with the result that mechanical properties, particularly abrasion resistance, are less than optimal. Moreover, the physical and mechanical properties of the cemented carbide support near the diamond/carbide interface are reduced as a result of cobalt depletion from the carbide support.
It is possible to control cobalt depletion from the cemented carbide support to some extent by placing a thin cobalt metal disc between the diamond layer and the carbide support prior to high pressure/high temperature processing. However, this solution does not avoid the infiltration of excessive cobalt into the polycrystalline diamond layer of the composite compact and the resulting diminished mechanical properties.
One attempt to resolve these shortcomings is described in U.S. Pat. No. 4,411,672, which provides a composite compact by placing a pulverized diamond layer adjacent to a tungsten carbide/cobalt layer, and separating these layers with a metallic material which has a melting point lower than the eutectic point of the tungsten carbide/cobalt composition. The assembly is heated at a temperature high enough to permit melting of the metallic material but which is insufficient to cause substantial melting of the tungsten carbide/cobalt composition. In this way, a controlled amount of metal is introduced into the pulverized diamond to promote bonding.
U.S. Pat. No. 4,440,573 describes another means to control the amount of metal which infiltrates from the carbide support into the polycrystalline diamond layer. The method of the '573 patent involves providing a mass of diamond particles and a mass of infiltrant metallic material, each mass having a substantially identical surface area. The mass of diamond particles and mass of infiltrant metallic material are positioned such that the surfaces are separated by a barrier layer of high melting metal having a surface area of 85% to 97% of the surface areas of said masses of diamond particles and infiltrant metallic material. The thus positioned masses and barrier layer are subjected to temperature-pressure conditions within the diamond stable region but below the melting point of the metallic barrier layer. In this way, a regulated amount of molten infiltrant metal is allowed to flow around the barrier layer and throughout the mass of diamond particles.
U.S. Pat. No. 4,764,434 teaches that a thin continuous layer of titanium nitride applied by chemical vapor deposition or physical vapor deposition to the carbide support material is sufficient to prevent diffusion of cobalt into the diamond table and thereby prevent embrittlement of the surface of the carbide support nearest the diamond table. According to the '434 patent, such thin titanium nitride layer acts as an effective diffusion barrier, preventing depletion of binder metal, such as cobalt, from the cemented carbide support.